1. Field of the Invention
The invention relates to a process for the production of catalytic converters for purifying exhaust gases, and more particularly to a method for producing catalytic converters exhibiting uniform mat densities, which is insensitive to substrate, supporting mat and metal shell variation.
2. Description of the Related Art
As is well known, the purification of exhaust gases from internal combustion engines, particularly in motor vehicles, is generally achieved by an exhaust gas purification system in which a ceramic element having a honeycomb cell structure acts as a catalyst carrier. More precisely, this honeycomb cell structure is covered with a catalyst that contains a precious metal which functions, in the presence of O2, to convert noxious components of the exhaust gas, such as HC and CO, to CO2 and H2O. The honeycomb cell structure is housed within a gas-tight, sheet metal or cast-metal heat resistant housing or can.
Honeycomb structures currently employed are typically comprised of a ceramic material such as cordierite; a brittle material exhibiting limited mechanical strength. The successful and practical use of these honeycomb ceramics as a catalytic converter substrates requires that the ceramic monolith be compressively held in a surrounding metal can, with Type 409 stainless steel being preferred. Additionally, catalytic converters in use today, typically include wrapping the monolith with a flexible, high temperature mat material and then enclosing it, utilizing one of several methods, within the metal can. In other words, the metal can is closed on the wrapped ceramic which combines with the cushioning mat to compressively load the substrate. The flexibility of the mat allows the supporting system to accommodate the irregular shape of the ceramic monolith and produce a continuous supporting system. The wrapped substrate protects against axial movement by the frictional component of the compressive pressure exerted by the can through the mat. It is held in place against radial movement by the compressed bulk of the mat. One typically sees that the axial holding power of the can/mat combination is a key measure of the suitability of the system design in an environment of high temperatures and mechanical vibration.
Within the field of exhaust systems, there are two major positions where the catalytic converter can be located. In a majority of automotive four-stroke engine applications, the converter is positioned downstream of the engine exhaust manifold as a stand-alone component. In this case, the shell of the converter is surrounded by ambient air. As such, the free moving air cools the exterior of the converter resulting in the exterior portion of the catalytic converter exhibiting a temperature significantly below the range of 500 to 650xc2x0 C., while at the same time the ceramic substrate experiences temperatures ranging from 800 to 950xc2x0 C. Since the temperatures to which the can is exposed to remain below 650xc2x0 C., a vermiculite-based intumescent mat, which generally expands as the temperature rises, is sufficient. This counteracts the loosening effect of the can""s expansion away from the ceramic. On the other hand, in a minority of automotive applications, the converter is placed very close to the engine manifold, where the temperatures on the mat generally exceed the generally accepted 650xc2x0 C. limit of vermiculite materials. In these applications, non-intumescent mat materials are generally preferred.
In another large class of vehicles, notably motorcycles and motor scooters, the converter is most often located within the muffler. Space is at a premium on motorcycles and scooters, and the catalytic converter is designed into the vehicle without changing the external conformance of the bike; this is true for both 2-stroke and 4-stroke engines. The thermal and mechanical conditions for these applications are more severe than for the above auto configurations as temperatures typically exceed 750xc2x0 C. or above. Pressure waves and vibration from the engine exhaust valving are more extreme. Engine rpm is higher than seen in autos; this creates a higher frequency vibrational environment. The 2-stroke exhaust waves are particularly severe since the exhaust valving occurs in the power stroke and is overlapped with the intake at the end of the piston downstroke. Further, the external surface of the motorbike shell is typically surrounded by hot engine exhaust within the muffler rather than a cooling ambient air stream such as seen in automotive installations. It follows that vermiculite based mat systems are unacceptable, and a non-intumescent vermiculite-free mat composition is needed. In sum, design of the motorcycle catalytic converter is made more difficult because temperatures are higher and expansion of the metal shell container cannot be counteracted by an intumescent material, as well as because of the fact that the vibration/shock effects are more severe.
Ceramic fiber mats, capable of exposure to temperatures as high as xcx9c1200xc2x0 C., represent an alternative to intumescents. The force generated by these mats is developed completely from the compression it undergoes during the canning of the catalytic converter. As such, the form of canning is critical to these fiber-based mats.
Stuff mounting is one method of canning that has been utilized in the past. Initially, the substrate is wrapped with the mat and inserted into a conical device that compresses the mat as it is pushed through. The wrapped substrate is then ejected from the compression cone into a cylindrical tube that serves as the converter container or shell. In the process of performing this activity, the mat must be maintained within a very narrow dimensional gap (high gap bulk density) between the can and the substrate. A major problem of this stuff mounting method is the inability of the process to compensate for variabilities in the mat basis weight, substrate diameter and in the metal shell container diameter. Even if the variabilities could be compensated or overcome, current techniques for stuff mounting these fiber based mats, at such high gap bulk densities, are, at best, inefficient processes.
Tourniquet style canning techniques have been developed which are capable of compensating for mat, substrate and container/can variability; i.e., techniques which allow the center region of the can to vary about nominal as the mat weight basis and ceramic diameter vary. For example, see U.S. patent application Ser. No. 09/130,172, which discloses a method for producing the catalytic converter which involves compressively closing the metal shell container around the supporting mat-wrapped honeycomb substrate using an optimized mat density. As disclosed, the tourniquet strap force produces a consistent and optimized pressure upon the mat, thereby allowing the resulting final can diameter to increase and decrease as the ceramic diameter and mat weight basis varies; i.e., the constant canning force achieves consistent mat pressure and variations in can wall thickness, substrate diameter and mat weight result in very small changes in this mat pressure. In other words, mat pressure is maintained at an optimum range without individual measurements of the components. Although this technique is effective for achieving larger automotive diameter catalytic converters exhibiting a mat compression of sufficient uniformity, difficulties arise for smaller motorcycle-size diameter catalytic converters where the force required for the tourniquet to bend the container wall (as opposed to compressing the mat) is a significant portion of the overall closure forces. Furthermore, uniformity suffers, especially at the tourniquet lap where a double thickness of can causes the can to be less flexible resulting in a flatter can curvature and localized increases in the mat density. As such, even though this tourniquet process produces small motorcycle catalytic converters exhibiting sufficient overall mat compression uniformity, a process is needed which is less complicated and is less labor intensive; i.e., a process which eliminates the need for welding to fix the container to the desired mat compression.
As such there remains a need for, and it is thus an objective of this invention to provide, for a simpler, less labor-intensive, more efficient canning process which achieves a uniform mat density, uniform compression on the ceramic substrate, in a manner such that the average mat density of a particular assembly is insensitive to variations in the average value of incoming parts, including the weight basis of the supporting mat, the diameter of the ceramic substrate and the thickness of the metal container.
It is therefore an objective of the present invention to disclose a formation method that overcomes the problems and shortcomings of the current compressive closing methods for forming catalytic converters. In other words, the present invention discloses a method of forming catalytic converters which achieves a compressive load upon the honeycomb structure which is sufficient to retain, but not damage the retained honeycomb substrate, and which is relatively insensitive to variations in supporting mat weight basis, ceramic substrate diameter and metal container thickness.
Forming a catalytic converter utilizing a compressive closing method generally involves wrapping the substrate in a sufficient amount of supporting mat material and inserting the wrapped substrate into a generally cylindrical metal container and thereafter compressively closing the container around the wrapped substrate sufficiently to provide a gas tight seal.
The present invention provides an improved method for forming a catalytic converter involving compressively closing the container around the wrapped substrate by resizing the container over substantially the entire portion of its length which is occupied by the wrapped substrate to a predetermined metal container outside diameter OD. The predetermined metal container outside diameter is characterized by the equation OD=D+2T1+2T2, wherein D is a diameter measure D of the substrate, T1 is the supporting mat target thickness and T2 is a container wall thickness measure.